The present invention relates to a polymerizable printing ink that is suitable for use with inkjet printers.
Polymerizable inks are well known in the printing industry. These inks are cured into thin, tough, abrasion-resistant films after printing by causing the polymerization of a monomer incorporated within the ink. Conventional polymerizable inks include a solvent, at least one monomer or oligomer, an initiator, various additives, and a pigment or dye. The monomer or oligomer is sometimes referred to as a pre-polymer. In the polymerization process, individual molecules of monomer are chemically bonded together to form a large polymer molecule. For example, polymerization of the monomer styrene (C.sub.6 H.sub.5 CH.dbd.CH.sub.2) yields the polymer polystyrene (--(C.sub.6 H.sub.5 CHCH.sub.2).sub.n --). A polymer may be a homopolymer, which is formed from a single monomer, or a heteropolymer, which is formed by the copolymerization of two or more different monomers. Polystyrene is an example of a homopolymer. An example of a heteropolymer would be styrene/butadiene, formed by the copolymerization of styrene and butadiene.
Initiators are molecules that are used to initiate a polymerization reaction. Generally, an initiator is a molecule that will decompose into a free radical-containing species upon the application of some form of energy. The free radical-containing species then attacks an unsaturated bond in a monomer molecule. This step both bonds the initiator to the monomer and converts the monomer into a free radical, which is then available to attack another monomer molecule. The resulting chain of free radical reactions is the polymerization process.
Various types of initiators are available. For example, a photoinitiator is a molecule that decomposes into a free radical-containing species upon the absorption of a photon of radiant energy, generally ultraviolet light. An example of a commonly used photoinitiator is benzophenone. A thermal initiator decomposes into a free radical-containing species upon the application of heat. An example of a thermal initiator is the water-soluble compound potassium persulfate. The persulfate ion (S.sub.2 O.sub.8.sup.2-) decomposes into two sulfate ion free radicals (SO.sub.4.cndot..sup.-) upon heating, both of which can then initiate a polymerization. The polymer chain grows in one direction away from the initiating species such that the polymer is terminated on one end by this species. Thus, polymers initiated with potassium persulfate are terminated on one end by a sulfate group. The polymerization reaction continues outward from this group until either all of the monomer is consumed or the free radical reacts with another free radical to terminate polymerization.
Various additives well known in the art are typically included in a photopolymerizable ink to solve problems associated with a particular ink formulation. For instance, some degree of polymerization may occur in an ink before it is applied to a substrate. This can both increase the viscosity of the ink and cause premature gelation. To remedy this, polymerization inhibitors such as antioxidants can be added to prevent premature polymerization and thus increase the shelf life of the ink. Another potential problem with an ink is the beading of an ink on a substrate surface caused by too high a surface tension in the ink. Beading prevents the ink from coating the substrate and results in lowered printing resolution. To prevent this, a wetting agent may be included to lower the surface tension of the ink and increase its tendency to coat the substrate. Additives are also added to inks to impart desired physical characteristics to an ink. For instance, if it is desired that the ink be flexible after it is cured so that it does not crack or peel from the substrate when the substrate is somehow deformed, a plasticizer can be added. If a faster curing time is desired, an accelerator may be included to increase the rate of polymerization of the ink. Also, it is important that the cured ink film bond well to the substrate. The addition of an adhesion promoter can be used to strengthen the adhesion of the ink film to a substrate when necessary. This list of additives is not exclusive; others may be added to the ink to give it any desired physical properties.
A coloring agent, either a dye or pigment, is dissolved or suspended in the solvent to give the ink color. Though there is no generally accepted distinction between dyes and pigments, most pigments are insoluble inorganic powders, while most dyes are insoluble, synthetic organic compounds. The coloring effect from pigments is a result of their dispersion in a liquid or solid medium. Because they are insoluble, an even dispersion of pigment in the medium is difficult to achieve. A pigmented ink must be stirred vigorously before use to ensure the best pigment dispersion possible. Stilling can not ensure that the pigment will remain well dispersed, however, because the pigment may settle out of the ink between printing and curing. Also, the particle sizes of the pigment fragments in an ink are non-uniform. Because the color intensity in a given region of a sample of a pigmented ink is directly proportional to the total amount of pigment present in that part of the ink, inconsistencies in color may occur in even a well-dispersed ink due to a non-uniform particle-size dispersion in the ink.
The use of a dye solves the problem of color inconsistencies due to different pigment particle sizes because the dye is dissolved in the ink solvent. Moreover, a wide variety of bright colors can be achieved by using organic dyes in an ink. However, the organic dyes tend to be less stable than pigments when exposed to environmental factors such as sunlight and chemical agents. Photobleaching, or the fading of the color of an ink when exposed to light, is a particular problem with photopolymerizable inks, as the ink must be exposed to dye-degrading ultraviolet light to initiate the polymerization reaction.
Many photopolymerizable ink mixtures are known. Bolon et al., U.S. Pat. No. 3,957,694; Batt et al., U.S. Pat. No. 4,137,138; Sakiyama et al., U.S. Pat. No. 4,221,686; Kurpiewski et al., U.S. Pat. No. 4,780,487; Lucey, U.S. Pat. No. 5,180,757; Amon et al., U.S. Pat. No. 5,658,964; and Erickson et al., U.S. Statutory Invention Registration No. H1517, all disclose photopolymerizable ink compositions for various uses. However, these inks are still subject to many of the difficulties discussed above, and none attempt to give superior protection to dyes against photobleaching or to disperse inorganic pigments more thoroughly. Additionally, further problems must be addressed when an ink is to be used in an inkjet printing process.
Inks must have certain flow and deformation characteristics under stress and strain, collectively known as Theological characteristics, to be to be suitable for use with an inkjet printing system. The particular rheological properties desirable in an ink may differ between printing techniques. For instance, it may be desirable for an ink to display either Newtonian or non-Newtonian viscosity characteristics. A Newtonian fluid, such as water, has a constant coefficient of viscosity for any given shear stress. Because of this, the rate of flow of a Newtonian fluid is directly proportional to the shear stress applied to the fluid. A non-Newtonian fluid has a variable coefficient of viscosity, which is referred to as the "apparent viscosity" at a given shear stress. For some applications it may be desirable for an ink to be a Newtonian fluid. Due to its constant coefficient of viscosity, a Newtonian ink may flow through an inkjet nozzle with less turbulence, and may be more adaptable to inkjet nozzles of different shapes and sizes.
On the other hand, it may be desirable for an ink to display some non-Newtonian behavior. For instance, it is sometimes desirable for an ink to have the non-Newtonian characteristic of thixotropy. A thixotropic material has an apparent viscosity which increases to a maximum value when the material is either at rest or subjected to a constant force, and decreases to a minimum value when it is subjected to a changing force. As an example, a thixotropic ink would have a low apparent viscosity when it is accelerated out of the inkjet orifice and when it strikes the paper. Once at rest on the paper, however, the ink would have a higher apparent viscosity, preventing it from running on the paper and preserving the resolution of the print. U.S. Pat. No. 5,024,700 to Britton, Jr. discloses a printing ink containing a thixotropic compound. A property similar to thixotropy is yield stress. A material with a yield stress requires the application of some minimum force before it will begin to flow. Thus, an ink with a significant yield stress would also resist running on paper after printing.
Suspensions of pigments in solvents may cause the solvent to unpredictably exhibit undesirable rheological characteristics. A potential cause of this problem is the morphology of the pigment particles. As mentioned earlier, the pigment particles may have a non-uniform size distribution. Furthermore, the pigment fragments may be irregularly shaped. The frictional interactions between pigment fragments in an ink heavily loaded with such a pigment may be inconsistent, and the particles may randomly aggregate and redisperse. Such interactions may prevent the solution from having predictable viscosity or flow characteristics, and thus may cause unpredictable behavior.
A fluid containing high concentrations of substantially uniform particles, on the other hand, may still demonstrate Newtonian behavior. In particular, it has been shown that suspensions of monodisperse polymer microspheres demonstrate such behavior. See Fukada, et al., Biorheology 26, 401-03 (1989) for a discussion of the rheological characteristics of aqueous suspensions of polymer microspheres. A polymer microsphere is a very small diameter, highly symmetrical sphere of a polymer such as polystyrene or polyvinyltoluene. Typical diameters of polymer microspheres range from 0.1 micrometer to 100 micrometers. Monodisperse polymer microspheres are microspheres having a substantially uniform and narrow size distribution. Polymer microspheres have many uses, including the use as a standard for electron microscopy calibration, blood flow modeling, latex agglutination tests in the field of immunodiagnostics, and various uses in phagocytosis research. A more complete list of uses for polymer microspheres is given by Leigh B. Bangs, Uniform Latex Particles, Seradyn, Inc., Particle Technology Division, 4.sup.th printing November 1987, p. 47-58.
Several different synthetic techniques can be used to produce polymer microspheres, depending upon the size of the microsphere product desired. For microspheres larger than 2 microns a multi-step swollen emulsion polymerization can be used. For microspheres smaller than about 2 microns, either an emulsification polymerization or an emulsifier-free polymerization can be used. The emulsification polymerizations are described in the Bangs article, Uniform Latex Particles, mentioned above. The single-step emulsification polymerization is performed by first mixing an aqueous surfactant solution so that the surfactant molecules form micelles. Next, a monomer such as styrene is mixed into the solution. Most of the monomer enters and swells the micelles, but some stays in the aqueous phase. To initiate the polymerization of the monomer dissolved in the micelles, a water-soluble initiator such as potassium persulfate is added to the solution. The solution is heated to generate sulfate ion free radicals, which then react with the aqueous styrene to initiate polymerization. The growing polystyrene chains in the aqueous phase soon migrate to the micelles, where their hydrocarbon ends are more soluble. The reaction continues in the micelles until either the chain reacts with another free radical or all of the monomer in the micelles is used up. Thus, when the chain reaction is complete, the polymer molecules will occupy the area inside the micelles formerly occupied by the monomer. The size of the spheres can be controlled by the choice of surfactant used.
Polymer microspheres have a surface charge resulting from the presence of charged groups on the microsphere surface. The electrostatic repulsion of these like-charged surface groups prevents aggregation of the microspheres. To do so, however, the repulsive forces must be sufficiently strong to overcome the attractive van der Waals forces between the microspheres. The primary source of these surface groups on emulsification-polymerized microspheres is the residual surfactant adsorbed to the surface of the microsphere. Intrinsic surface groups originating from the initiator are also present in a lesser quantity. If an ionic initiator was used for the polymerization, the end group of the polymer will be charged and will remain on the microsphere surface when the growing polymer chain migrates from the aqueous phase to the micelle interior. As an example, a microsphere synthesized in a sodium dodecyl sulfate solution (a common surfactant) using potassium persulfate as an initiator will have both intrinsic sulfate surface groups originating from the initiator and sulfate surface groups belonging to the surfactant.
The emulsification polymerization synthesis is not the preferred synthesis for smaller microspheres, however. In order to control the colloidal stability of polymer microspheres to prevent aggregation, it is desirable to have the ability to closely control the surface charge density of the microspheres. Aggregation of microspheres can disrupt the flow characteristics of the microsphere suspension. See Fukada, et al., Biorheology 26, 401-03 (1989), for a discussion of the effects of aggregation on the flow characteristics of a microsphere suspension. To control the surface charge on a microsphere, it is desirable to add a precise, known quantity of surfactant to the surface of each microsphere. The emuslification polymerization process, however, does not allow the ready control of the density of the surface electrical charge of the microspheres. Instead, the surfaces of microspheres synthesized by the emulsification process typically exhibit a variable surface charge density. Because of this, the surfactant from the emulsification process must be removed by washing or ion exchange so that, if needed, a known quantity of surfactant can then be added to the microsphere surfaces. The surfactant removal is difficult to perform to completion, and can itself cause aggregation, because the number of intrinsic surface groups present on these polymer microspheres is too low to counteract the attractive forces between microspheres without the assistance of the surfactant.
Better results are achieved when an emulsifier-free polymerization is used to synthesize small polymer microspheres. An emulsifier-free polymerization yields microspheres with a higher concentration of intrinsic surface groups so that the microspheres are stable in suspension even without additional surfactant. Moreover, the surface charge density of these microspheres is easily adjusted in a controlled fashion by the addition of a surfactant to the microsphere solution after the synthesis is complete. An example of an emulsifier-free synthesis, given in Goodwin, et al., Colloid & Polymer Sci. 252, No. 6, 461-74 (1974), is as follows: First, styrene is dissolved in water. Next, an initiator such as potassium persulfate is mixed into the solution, and the solution is heated. Small surface active oligomers form, which grow, associate, nucleate, and form particles which subsequently become swollen with monomer. The hydrophilic sulfate groups from the initiator stay on the surface of the nucleating particles. Eventually, enough charged surface groups form on the growing particle to give the particle an adequate electrostatic surface potential to be colloidally stable. At this point, the particle has enough intrinsic surface groups to repel other particles, and the polymerization continues within the particle until the microsphere is complete. The diameters of the microspheres formed can be controlled through careful manipulation of the ionic strength of the aqueous solution, the monomer concentration, the initiator concentration, and the reaction temperature. This synthesis provides microspheres with both an adequate surface potential to be colloidally stable in water without any added surfactant, and a surface free of the residual surfactant contaminants that are produced by other synthetic methods. After synthesis, the surface charge density of the microspheres can be accurately tailored to suit a particular purpose. The effects of ionic strength, monomer concentration, initiator concentration, and temperature on microsphere diameter are discussed in the Goodwin article mentioned earlier.
Pigments and dyes, collectively referred to as coloring agents, can be added to microspheres. The coloring agent can either be chemically bonded to the outside of a microsphere, chemically included within the microsphere, or can completely permeate the microsphere by being both bonded to the outside and included within. Either inclusion of the coloring agent within the microsphere or complete permeation is preferred because a much greater number of coloring agent molecules or particles can be added to each microsphere, resulting in more intense colors. A process for dyeing microspheres is described in Uniform Latex Particles by Bangs, cited above. As described in this article, the process of dyeing the microspheres involves first preparing a dilute suspension of microspheres in water. Next, an oil-soluble dye is dissolved in an essentially nonpolar organic solvent. The polarity of a solvent is an expression of the net dipole moment, or the degree of asymmetry in the electron charge density, of the solvent molecules. An essentially nonpolar solvent has little to no net dipole moment, and dissolves nonpolar molecules, such as many dye molecules, that will not dissolve in a polar solvent, such as water. After dissolving the dye in the organic solvent, the organic solvent-dye mixture is added dropwise to the aqueous microsphere suspension while gently stirring the suspension. The organic dye mixture swells the microspheres and incorporates the dye within the microspheres. After maximum dye loading has been achieved, the organic solvent is distilled out of the solution using a rotary evaporator at an elevated temperature.
The coloring process disclosed in the Bangs article, however, is far from ideal. The addition of the organic solvent-dye mixture to the aqueous phase must be performed very slowly or it will not work. It is thus very time consuming, and can take hours or even days. Moreover, if the organic phase is added to the aqueous phase too quickly, the microspheres will actually dissolve in the organic phase, resulting in the formation of large, sticky globules of dye/solvent/polymer and ruining the microsphere suspension. There is always a chance that this will happen even if the process is performed slowly and carefully. Thus, this process is not suitable for the large-scale production of colored polymer microspheres.
Accordingly, it is a general object of the present invention to provide an improved polymerizable ink. More specific objects of the present invention are to provide an improved pigment for use in an ink and a method for making the same, to provide a polymerizable ink that offers superior protection from photobleaching to dyes, to provide an ink that disperses pigments uniformly without stirring, to provide an ink for use with inkjet printing methods that has highly controllable and predictable rheological characteristics, and to provide methods for making and using an ink with the above qualities.